1. Field of the Invention
The present invention relates to a tool holding apparatus having a cutting edge moving function and, more particularly, to a tool holding apparatus which can detect a moving amount of a cutting edge of a tool mounted on the holding apparatus.
2. Description of the Prior Art
A known tool holding apparatus (so-called boring head) comprising a main body with a shank portion for fixing on a machine spindle, and a tool slide slidable in a direction perpendicular to the main body axis is known. In this boring head, the slide is moved in the direction perpendicular to the main body axis by rotating a feed screw assembled in the main body, and the position of the cutting edge of a tool fixed on the slide is displaced. (Japanese Pat. Publication No. 42-18519, U.S. Pat. Nos. 3,800,637, 3,371,559, 2,771,798 and others).
In such a holding apparatus, the moving amount of the cutting edge of the tool mounted on the slide is determined in accordance with the rotation amount of the feed screw. However, with this cutting edge moving amount detecting method, an error occurring due to a backlash of the feed screw cannot be absorbed. For this reason, in a conventional apparatus of this type, a graduated scale is formed in a side surface of the main body or slide, and the moving amount of the cutting edge is directly read in accordance with the scale.
However, with this detecting means utilizing a calibrated scale, since the minimum scale unit is 1 mm, small displacements cannot be measured. In addition, small chips or cutting oil attaches to the scale, and precise readings may not be obtained.
Since the scale is fixed on the main body or the like, calculation is required to determine a relative displacement of the cutting edge. For this reason, detection is cumbersome and a calculation error tends to occur.
In order to solve these problems, the scale can be digitized. However, since this type of apparatus is mounted on a machine tool, chips or cutting oil tend to attach to the electrodes or circuit portions constituting the digital scale. For this reason, it is difficult to obtain digital measurement signals with high precision without causing an erroneous operation. Thus, a detecting means using a graduated scale as described above must be used despite its drawbacks.